I hate being threatened too! I think it's a natural response to what your body and mind perceive as a, well threat. The adrenaline kicks in, etc.

However, if I were to put on my Psychologist's hat (I'm not one, but I'm put the hat on anyway), I would suggest some of it has to do with your step father, who you previously wrote about. He was an unpleasant character and I'm guessing you may have often felt powerless as a kid in the face of his threatening behaviour. There could be something about threats now that triggers that in some way. As in, something in you consciously or not remembering and not liking that helpless feeling and sort of rallying against it in a way to never let anyone ever make you feel powerless again. Of course, that's just my observation, I'm no psychologist, like I said, I'm only wearing the hat. ;)